And this isn't reversible is it?
Not reversible... and
There is no set rule on double displacement and/or single displacement being exothermic or endothermic. Exothermic vs. endothermic depends on the individual molecules and the transformation taking place in each individual reaction.
To figure out if a reaction is exothermic or endothermic, you have to evaluate the heat of the reaction. This is done by factoring the heat of formation of the products and the heat of breaking apart the reactants into your calculation.
The quantity you will evaluate for each reaction is dHrxn (the d stands for delta).
If dHrxn < 0, the reaction is exothermic
If dHrxn > 0, the reaction is endothermic
dHrxn = sum dHf(products) - sum dHf(reactants)
Hf is the heat of formation of a substance and can be found in a table, there should be one in your chemistry text or they may be found from a reliable source online. The quantities are usually given in a per mole basis, so you multiply by the number of moles in your given reaction to the quantity you need.
Example: Is the following reaction endothermic or exothermic?
8 Al(s) + 3 Fe3O4(s)
4 Al2O3(s) + 9 Fe(s)
Step 1: Find dHf(products)
from a table
Hf Al203 = -1669.8 kJ
Hf Fe = 0 because it is in its natural state
so dHf(products) = 4 x (-1669.8) = -6679.2
Step 2: Find dHf (reactants)
Hf Fe3O4 = -1120.9 kJ
Hf Al = 0 because it is in its natural state
dHf (reactants) = 3(-1120.9) = -3362.7
dHrxn = dHf (products) - dHf (reactants)
= -6679.2 - -3362.7
= -3316.5 kJ
dHrxn < 0, so the reaction is exothermic (it gave off heat to the environment)
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